An image forming system includes an image forming apparatus and an insertion device. The image forming apparatus includes an image forming device, a transport device, and a control device. The control device acts as a controller and an executer. The controller causes the image forming device to execute a printing job, and causes the insertion device to insert the interleaving paper in a printed material at an insertion start timing designated by the printing job. The executer executes calibration at an execution start timing set in advance, and sets a time point that a predetermined time has elapsed after a previous calibration to the execution start timing in advance. The executer starts to execute the calibration at the insertion start timing, when a predetermined condition is satisfied.